The present disclosure relates to a curing agent composition for an epoxy resin, more particularly, to a liquid curing agent composition comprising a polyamine and an accelerator dicyandiamide, the amine-epoxy resin composition, and the cured products from amine-epoxy composition.
Epoxies are known for their excellent adhesion, chemical and heat resistance, good-to-excellent mechanical properties and very good electrical insulating properties. Cured epoxy resins have found extensive applications in coatings, adhesives, and composite materials such as those using carbon fiber and fiberglass reinforcements. The chemistry of epoxies and the range of commercially available variations offer a broad range of properties tailored for various specific applications. Some specific examples of the applications are listed below. For examples, cured epoxy resins are used as laminating resin for airframe and missile applications, for filament-wound structures, for resin transfer molding, and for tooling fixtures; as casting resin for molds, dies and tooling; as potting and encapsulation compounds, as impregnation resins, as adhesives for aircraft honeycomb structures, as body solders and caulking compounds for repair of plastic and metal parts, and as caulking and sealant compounds in building and highway construction and in applications where high chemical resistance is required. Epoxy-based coatings are used as maintenance and product finishes, marine finishes, masonry finishes, structural steel coatings, tank coatings, aircraft finishes, appliance primers, automotive primers, can and drum linings, furniture finishes, and as varnishes for electric and electronic equipment. They are also used for concrete-floor paints, gym and floor varnishes.
Composite materials are materials made from two or more constituent materials, i.e. matrix and reinforcement, with significantly different physical or chemical properties, that when combined, produce a material with characteristics different from the individual components. The matrix material surrounds and supports the reinforcement materials by maintaining their relative positions. The reinforcements impart their special mechanical and physical properties to enhance the matrix properties. A synergism produces material properties unavailable from the individual constituent materials, while the wide variety of matrix and strengthening materials allows the designer of the product or structure to choose an optimum combination. The reinforcement materials are often fibers such as glass fibers, carbon fibers, or high strength polymers for example, Kevlar® fibers. For polymer composites, the matrix is polymer-based. The most common polymers used in polymer composites are epoxy, phenolic, polyester, polyimide, and others.
Polymer composites offer several advantages compared to metals and ceramics in that polymer composites are lightweight, have high specific stiffness and strength; are easy to manufacture, allow tailoring of the properties by varying the resin's chemistry, reinforcement fibers, and design flexibility for different applications and also have low coefficients of thermal expansion.
Polymer composites, in particular thermosetting polymer material such as epoxy resin, prepared through crosslinking reaction with an appropriate curing agent, based on amine or polyamine(s), have the following desired properties: (a) low to high curing temperature, where the curing reaction can be carried out at a wide range of temperatures from 5° C. to 150° C.; (b) low volume shrinkage rate, where the volume shrinkage rate of cured epoxy resin is typically 1 to 3% resulting in low internal stress in fiber matrix composites; (c) good wetting, adhesion to provide good shear strength between fiber and matrix; (d) good insulating properties; (e) good chemical resistance; and (f) good thermal properties. There are many types of composite applications such as prepregs, laminates, filament windings, braiding, pultrusion, wet lay-up and infusion composites. Resin infusion, or resin transfer, is a process by which resin is introduced to the composite mold, the reinforcement material having already been placed into the mold and closed prior to resin introduction. Conventional processing and chemistry cannot meet current throughput requirements. It is necessary to use additives to improve the cycle time without sacrificing processing and cured product properties.
Epoxy resin compositions often consist of a conventional epoxy resin or epoxy resin mixture, a curing agent and possibly a curing accelerator, and optionally a solvent or a solvent mixture to adjust the viscosity or solubility.
Dicyandiamide has long been known as a latent curative for epoxy resins. (H. Lee and K. Neville “Epoxy Resins” McGraw Hill, New York, 1967, pages 10-16). Because dicyandiamide is not very soluble in either epoxy resin or curing agent, dicyandiamide in general is dispersed in an epoxy resin to provide a one component epoxy with a shelf life of at least 6 months. U.S. Pat. No. 3,391,113 discloses the uses of dicyandiamide as a curing agent for epoxy resin and tetraalkylguanidines as an accelerator for dicyandiamide. EP 659833 discloses an epoxy resin composition comprising an epoxy resin and a curing agent consisting of dicyandiamide, a cycloaliphatic polyamine, a polyoxyalkylene amine and a cure accelerator of tertiary amines, quaternary ammonium compounds, or alkali metal alkoxides. U.S. Pat. No. 5,214,098 discloses the uses of dicyandiamide as a latent curing agent for an epoxy resin composition which cures at temperature of at least 80° C. The composition also includes an amine and a thiol and a cure accelerator of imidazole or substituted imidazoles. US2005/0137357 A1 discloses a curing agent composition comprising a polyamine, a polyamide, dicyandiamide and an imidazole for an epoxy adhesive composition. Dicyandiamide is dispersed in the epoxy resin.
In the art epoxy curing agent listed above, dicyandiamide is in general dispersed in epoxy resin, used as a latent curing agent in conjunction with an accelerator such as an imidazole derivative. Since dicyandiamide cured epoxy systems process high impact strength and abrasion resistance, excellent adhesion to substrates, and good stability towards water and chemicals, effort has also been directed to prepare a homogeneous curing agent solution to incorporate dicyandiamide to maximize its advantage.
U.S. Pat. No. 4,859,761 discloses the preparation and use of soluble substituted cyanoguanidines as curing agents for epoxy resins. Curing of the epoxy resin-substituted cyanoguanidine system is carried out at temperatures of 100-300° C. with an imidazole accelerator.
U.S. Pat. No. 4,621,128 discloses a combination dimethylformamide and a low boiling ketone such as acetone or methyl ethyl ketone, and/or alcohol to solubilize dicyandiamide to prepare a homogenous curing agent that cures epoxy above room temperature.
U.S. Pat. No. 3,420,794 discloses a process for curing an epoxy resin using a curing agent comprising dicyandiamide dissolved in an amine such as diethylenetriamine or N-aminoethylpiperazine.
EP2180012A1 discloses a curing agent solution for epoxy resin composition. The disclosed curing agent solution comprises dicyandiamide dissolved in an amine such as isophorone diamine or Jeffamine D230.
The disclosure of the foregoing publications including patents and patent applications is hereby incorporated by reference.
Due to the wide range of applications of epoxy in coating, adhesive, and composite field, the material requirements for each application varies. For epoxy composite, good adhesion and wettability to the reinforcement fibers are highly desirable to maximize the synergy between epoxy matrix and fibers. A homogeneous liquid curing agent that is compatible with epoxy resin is critical to obtain a homogeneous distribution of epoxy resin on the fibers. Many epoxy composites are processed by heating in order to cure the epoxy resin. Lowering the heat cure temperature, while achieving the same or higher degree of cure will reduce energy consumption, reduce processing time, and increase throughput.
Many composite applications require a high degree of resistance to heat and chemicals and high glass transition temperature (Tg). High Tg requires extensive crosslinking which usually tends to adversely cause embrittlement. It is desirable to produce a cured epoxy having a high Tg and improved chemical resistance while maintaining the mechanical properties.
For coating applications, good adhesion to substrate, improved cure speed, and fast hardness development is essential to ensure short overcoat time if used as primer or middle coat, and fast return to service. A curing agent that can deliver these properties to an epoxy system is highly desirable. In addition, epoxy coatings tend to yellow over time when exposed to light. The common accelerators used in the curing agent such as salicylic acid, or 2,4,6-tri(dimethyaminomethyl)phenol (available as Ancamine® K54 from Air Products & Chemicals, Inc.) can accelerate the yellowing process. A yellowing resistant curing agent composition is also highly desirable for coating applications. When used in adhesive application, good adhesion to substrates and strong bond strength is required.
In all applications, the decrease in the usage level of curing agent while achieving similar or better properties is highly desirable to reduce raw material cost thus reduce cost of the goods manufactured. This would require a better and faster curing agent compared to similar current products. Faster curing will also make it possible to apply coatings or adhesives at temperature below ambient and widen the application window in colder climate.
The need and improvements desired in the art of epoxy curing agents include homogeneous solution, fast cure speed, lower cure temperature, reduced usage level, yellowing resistance, improved chemical resistance, maintaining thermal and mechanical properties, and improved adhesion. These needs and improvements are addressed by the embodiments of the present disclosure as described below and defined by the claims that follow.